A method and system for terrain correction in near, medium and far areas of large-scale gravity exploration

Abstract

The present invention relates to the field of geophysical exploration, and discloses a method and system for correcting near, middle and far areas of large-scale gravity exploration, the method comprising: obtaining the total accuracy requirements of target Bouguer gravity anomalies and the three-dimensional coordinates of gravity measuring points in the exploration area; determining the most suitable near-area terrain correction model or model combination, and obtaining near-area terrain correction model parameters; reading the near-area terrain correction entry table and the point number of the gravity measuring point, and calculating the near-area terrain correction value of each gravity measuring point; obtaining DEM data of the exploration area and its neighborhood through public channels, and converting them into normal elevation DEM data with an elevation benchmark of quasi-geoid through a global high-order earth gravity field model; calculating the mid-to-far area terrain correction value of the gravity measuring point according to the converted DEM data combined with the three-dimensional coordinates of the gravity measuring point; and respectively evaluating the accuracy of the near, middle and far area terrain correction values. The present invention not only ensures the accuracy of the mid-to-far area terrain correction, but also solves the problems of difficulty in collecting DEM data and the existence of gaps.

Description

Technical Field

[0001] The invention relates to the field of geophysical exploration, and in particular to a method and system for correcting terrain in near, middle and far areas of large-scale gravity exploration. Background Art

[0002] Gravity exploration is one of the main methods of geophysical exploration. Its basic principle is to obtain the gravity variation field caused by the density difference of the crustal rocks through ground or aerial gravimeters, and to conduct qualitative and quantitative data processing and interpretation of the gravity variation field according to the research objectives. It is widely used in regional geological surveys, energy resource exploration, earthquake fault zones, deep crustal structures and other fields.

[0003] In recent years, with the great improvement in the observation accuracy and stability of relative gravimeters, large-scale (≥1:50,000) gravity exploration has become a key exploration method for directly prospecting ore bodies or indirectly prospecting ore bodies by identifying mineralized rock bodies and ore-controlling structures. It plays an important role in the fields of solid minerals, geothermal resources and groundwater exploration, shallow engineering geology and karst exploration.

[0004] Bouguer gravity anomaly is the basic data of gravity exploration. It is the data of the gravimeter observation at the measuring point after the closure of the morning and evening gravity base points, gravity normal field correction, terrain correction (near, middle and far area correction) and Bouguer correction (intermediate layer correction and height correction). Among them, terrain correction has an important influence on Bouguer gravity anomaly. Therefore, how to improve the accuracy of terrain correction is the key to obtaining high-precision Bouguer gravity anomaly data. In addition, efficient terrain correction is also a key factor in improving the efficiency of Bouguer gravity data collection.

[0005] Currently, near-region terrain correction can be divided into two categories. The first category is to use the near-region terrain correction model in the gravity exploration specifications or textbooks; the second category is to use laser terrain scanning devices, RTK near-region terrain multi-point elevation measurement, and drones to obtain digital elevation models (DEM data). Among them, the existing near-region terrain correction models involved in the first category are too simple and difficult to meet the needs of mountainous areas with large terrain fluctuations and complex and changeable stepped areas. The main problems with the second type of near-region terrain correction method are large workload, long time consumption, high labor costs, and vegetation interference factors that make it difficult to obtain effective elevation data.

[0006] At present, the mid-to-far-field terrain correction can be divided into two categories. The first category is the digital terrain obtained by drones; the second category is the collection of digital elevation model (DEM data, the elevation datum is the normal height of the quasi-geoid) data of the exploration area and the neighborhood in the surveying and mapping part. The calculation method of terrain correction in the mid-to-far-field area is to divide the terrain correction area into circular rings into fan-shaped columns. The height of each fan-shaped column is the difference between the average height of the digital terrain in the fan-shaped area and the elevation of the gravity measurement point. Among them, the first category has a large workload and is only considered when DEM data cannot be collected. The second category is the most commonly used process at present, but there are problems such as difficulty in data collection, data splicing, and gaps in certain waters. Summary of the invention

[0007] The purpose of the present invention is to propose a method and system for large-scale gravity exploration near, middle and far zone terrain correction, so as to solve the technical problems of poor applicability of the current large-scale gravity exploration near zone terrain correction model, difficulty in collecting DEM data for mid and far zone terrain correction, data splicing, and gaps in certain water areas.

[0008] The present invention provides a method for correcting terrain in near, medium and far areas of large-scale gravity exploration, comprising the following steps:

[0009] S1: Obtain the total accuracy requirements of the target Bouguer gravity anomaly and the three-dimensional coordinates of the gravity measurement points in the exploration area;

[0010] S2: Determine the most suitable near-area terrain correction model or model combination on-site at each gravity measurement point, and obtain the near-area terrain correction model parameters;

[0011] S3: Read the near-area terrain correction entry table and the point number of the gravity measuring point according to the near-area terrain correction model parameters, and calculate the near-area terrain correction value of each gravity measuring point;

[0012] S4: Obtain DEM data of the exploration area and its vicinity through public channels, and convert them into normal elevation DEM data with an elevation datum of quasi-geoid using the global high-order earth gravity field model;

[0013] S5: Calculate the mid- and far-area terrain correction value of the gravity measuring point based on the converted DEM data and the three-dimensional coordinates of the gravity measuring point;

[0014] S6: Evaluate the accuracy of terrain correction values ​​in near, middle and far areas of large-scale gravity exploration.

[0015] The beneficial effects provided by the present invention are:

[0016] 1. For the near-area terrain correction of large-scale gravity exploration, the problems existing in the existing methods of near-area terrain correction are fully considered. On the basis of the near-area terrain correction model in the gravity exploration specifications or textbooks, a new near-area terrain correction model that is more suitable for mountainous areas and terraced areas is obtained through theoretical formula derivation. The existence of the new near-area terrain correction model makes the selection of gravity measurement points more practical and principled, which can moderately reduce the impact of near-area terrain correction and improve the accuracy of near-area terrain correction. It greatly reduces the workload and improves work efficiency.

[0017] 2. For the mid-to-far-region terrain correction of large-scale gravity exploration, it is proposed to download the non-dense DEM data of the exploration area and its neighborhood from the official website open to the public, and convert it into normal elevation DEM data through the global high-order earth gravity field model EGM2008 for large-scale mid-to-far-region terrain correction calculation. This not only ensures the accuracy of mid-to-far-region terrain correction, but also solves the problems of difficulty in collecting DEM data, data splicing, and gaps in certain waters. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic flow chart of the method of the present invention;

[0019] Figure 2 It is a schematic diagram of an improved slope model of a method for correcting terrain in near, middle and far areas of large-scale gravity exploration in an embodiment of the present application;

[0020] Figure 3 It is a schematic diagram of a cone model for improving a method for correcting terrain in near, middle and far areas of large-scale gravity exploration in an embodiment of the present application;

[0021] Figure 4 It is a schematic diagram of a step model for improving a method for correcting terrain in near, middle and far areas of large-scale gravity exploration in an embodiment of the present application;

[0022] Figure 5 It is a schematic diagram of the mid-to-far zone terrain correction circle subdivision of a large-scale gravity exploration near-to-far zone terrain correction method in an embodiment of the present application;

[0023] Figure 6 It is a schematic diagram of different rotation angles of a method for correcting terrain in near, middle and far areas of large-scale gravity exploration in an embodiment of the present application;

[0024] Figure 7 It is a statistical diagram of the error between the normal height actually measured by a large-scale gravity exploration near, middle and far area terrain correction method and the normal height of the DEM data collected by the surveying and mapping department in an embodiment of the present application;

[0025] Figure 8It is a statistical diagram of the errors between the normal height actually measured by a method for correcting terrain in near, middle and far areas of large-scale gravity exploration and the normal height converted from the downloaded DEM in an embodiment of the present application. DETAILED DESCRIPTION

[0026] To make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0027] Before formally describing the present invention, the scheme of the present invention is first generally described for easy understanding.

[0028] First, the application scenario of the present application is introduced. The present application can be applied to the field of large-scale gravity exploration terrain correction technology.

[0029] Bouguer gravity anomaly is the basic data of gravity exploration. It is the data of the gravimeter observation at the measuring point after the closure of the morning and evening gravity base points, gravity normal field correction, terrain correction (near, middle and far area correction) and Bouguer correction (intermediate layer correction and height correction). Among them, terrain correction has an important influence on Bouguer gravity anomaly. Therefore, how to improve the accuracy of terrain correction is the key to obtaining high-precision Bouguer gravity anomaly data. In addition, efficient terrain correction is also a key factor in improving the efficiency of Bouguer gravity data collection.

[0030] Currently, near-region terrain correction can be divided into two categories. The first category is to use the near-region terrain correction model in the gravity exploration specifications or textbooks; the second category is to use laser terrain scanning devices, RTK near-region terrain multi-point elevation measurement, and drones to obtain digital elevation models (DEM data). Among them, the existing near-region terrain correction models involved in the first category are too simple and difficult to meet the needs of mountainous areas with large terrain fluctuations and complex and changeable stepped areas. The main problems with the second type of near-region terrain correction method are large workload, long time consumption, high personnel costs, and vegetation interference factors that make it difficult to obtain effective elevation data.

[0031] At present, the mid-to-far-region terrain correction can be divided into two categories. The first category is the digital terrain obtained by drones; the second category is the digital elevation model (DEM data, the elevation datum is the normal height of the quasi-geoid) data collected in the surveying and mapping part of the survey area and the neighborhood. The calculation method for terrain correction in the mid-to-far-region is to divide the circular ring area into fan-shaped columns. The height of each fan-shaped column is the difference between the average height of the digital terrain in the fan-shaped area and the elevation of the gravity measurement point. Among them, the first category has a large workload and is only considered when DEM data cannot be collected. The second category is the most widely used method at present, and it also has problems such as difficulty in data collection, splicing, and gaps in certain waters.

[0032] The embodiments of the present application fully consider the problems existing in the two existing methods of near-area terrain correction. Based on the near-area terrain correction model in the gravity exploration specification or textbook, a new near-area terrain correction model that is more suitable for mountainous areas and terraced areas is obtained through theoretical formula derivation. The existence of the new near-area terrain correction model makes the selection of gravity measurement points more practical and principled, which can moderately reduce the impact of near-area terrain correction and improve the accuracy of near-area terrain correction.

[0033] The embodiments of the present application fully consider the problems existing in the two existing methods for terrain correction in mid- and far-field areas. On the premise of ensuring the accuracy of terrain correction in mid- and far-field areas, it is proposed to download non-dense DEM data (the elevation datum is the geoid height of the WGS84 ellipsoid) of the neighborhood of the exploration area from an official website open to the public, and convert it into DEM data of normal elevation through the global high-order earth gravity field model EGM2008, which is used for large-scale terrain correction calculation in mid- and far-field areas.

[0034] Please refer to Figure 1 , Figure 1 It is a schematic flow diagram of the method of the present invention;

[0035] The present invention provides a method for correcting terrain in near, medium and far areas of large-scale gravity exploration, comprising the following steps:

[0036] S1: Obtain the total accuracy requirements of the target Bouguer gravity anomaly and the three-dimensional coordinates of the gravity measurement points in the exploration area;

[0037] It should be noted that in step S1, a real-time differential RTK measuring instrument is used to obtain the three-dimensional coordinates of the gravity measuring point.

[0038] As an embodiment, the present invention determines the total accuracy of Bouguer gravity anomaly of large-scale gravity survey according to the target task of gravity survey, designs the survey line and survey point of gravity survey according to the scale size and Bouguer total accuracy requirement, and uses a real-time differential RTK surveying instrument to obtain the three-dimensional coordinates of the gravity survey point. The three-dimensional coordinates of the gravity survey point include the horizontal coordinates X, Y and normal height h of the survey point.

[0039] It should be noted that before step S2, according to the Bouguer total accuracy and the near-area ground modification model parameter table, the near area within the preset area is regarded as flat land and no change is made.

[0040] As an embodiment, the near-area ground modification model parameter table can be referred to Table 1. In the present invention, when the circular area within 0-20m can be regarded as flat ground, the near-area ground modification is not required.

[0041] When conducting large-scale gravity exploration in mountainous areas, it is not easy to find measuring points that are flat within the circular range of 0-20m. At this time, the terrain undulation within 20m near the measuring point should be observed, and the near-area terrain correction model that best fits the actual terrain undulation should be determined and the model parameter data should be recorded.

[0042] Table 1

[0043]

[0044] Specifically, it is divided into the following situations:

[0045] (1) When the scale of gravity exploration is greater than 1:50,000 and the total accuracy requirement of the target Bouguer gravity anomaly is 0.050 mGal, the improved single slope model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the single slope is greater than 15 meters, which can be regarded as flat ground; the improved cone model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the cone model is greater than 13 meters, which can be regarded as flat ground; the improved step model of near-area terrain correction, when the height difference of the step is less than 10 meters, the distance from the measuring point to the edge of the step is greater than 15 meters, which can be regarded as flat ground.

[0046] (2) When the scale of gravity exploration is greater than 1:50,000 and the total accuracy requirement of the target Bouguer gravity anomaly is 0.100 mGal, the improved single slope model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the single slope is greater than 14 meters, which can be regarded as flat ground; the improved cone model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the cone model is greater than 11 meters, which can be regarded as flat ground; the improved step model of near-area terrain correction, when the height difference of the step is less than 10 meters, the distance from the measuring point to the edge of the step is greater than 13 meters, which can be regarded as flat ground.

[0047] (3) When the scale of gravity exploration is greater than 1:50,000 and the total accuracy requirement of the target Bouguer gravity anomaly is 0.150 mGal, the improved single slope model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the single slope is greater than 12 meters, which can be regarded as flat ground; the improved cone model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the cone model is greater than 8 meters, which can be regarded as flat ground; the improved step model of near-area terrain correction, when the height difference of the step is less than 10 meters, the distance from the measuring point to the edge of the step is greater than 9 meters, which can be regarded as flat ground.

[0048] (4) When the scale of gravity exploration is 1:50,000 and the total accuracy requirement of the target Bouguer gravity anomaly is 0.080 mGal, for the improved single slope model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the single slope is greater than 14 meters, which can be regarded as flat ground; for the improved cone model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the cone model is greater than 11 meters, which can be regarded as flat ground; for the improved step model of near-area terrain correction, when the height difference of the step is less than 10 meters, the distance from the measuring point to the edge of the step is greater than 13 meters, which can be regarded as flat ground.

[0049] (5) When the scale of gravity exploration is 1:50,000 and the total accuracy requirement of the target Bouguer gravity anomaly is 0.160 mGal, for the improved single slope model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the single slope is greater than 13 meters, which can be regarded as flat ground; for the improved cone model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the cone model is greater than 10 meters, which can be regarded as flat ground; for the improved step model of near-area terrain correction, when the height difference of the step is less than 10 meters, the distance from the measuring point to the edge of the step is greater than 11 meters, which can be regarded as flat ground.

[0050] (6) When the scale of gravity exploration is 1:50,000 and the total accuracy requirement of the target Bouguer gravity anomaly is 0.300 mGal, for the improved single slope model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the single slope is greater than 12 meters, which can be regarded as flat ground; for the improved cone model of near-area terrain correction, when the inclination angle is less than 70 degrees, the distance from the measuring point to the edge of the cone model is greater than 9 meters, which can be regarded as flat ground; for the improved step model of near-area terrain correction, when the height difference of the step is less than 10 meters, the distance from the measuring point to the edge of the step is greater than 10 meters, which can be regarded as flat ground.

[0051] S2: Determine the most suitable near-area terrain correction model or model combination on-site at each gravity measurement point, and obtain the parameters of the near-area terrain correction model;

[0052] It should be noted that the near-area terrain correction model in step S2 includes: an improved single slope model, an improved cone model and an improved step model.

[0053] In order to reduce the workload and improve the accuracy and work efficiency of near-area land modification, the existing near-area land modification models in large-scale gravity exploration specifications and textbooks are difficult to meet the actual terrain needs. The present invention derives a more applicable near-area land modification model based on the statistics of mountainous terrain and complex and changeable platform terrain.

[0054] Specifically, a distance parameter is added based on the original single slope model and cone model. Figure 2 As shown in Figure 1, this model is an improved single slope model. The gravity survey point is located at the origin of the coordinate system, 0, where r = 0 is the original slope model. The laser instrument is used to measure the distance r between the observation point and the slope and the inclination angle α of the slope.

[0055] The calculation formula of the single slope model is as follows:

[0056] (1)

[0057] in, is the gravitational constant 6.67×10-11 m3 / (s2· kg), ρ is the terrain-corrected density 2.67g / cm3, represents the distance from the observation point to the slope, It represents the inclination angle of the slope, and r1 represents the maximum radius of the near-area terrain correction.

[0058] like Figure 3 As shown in the figure, this model is an improved cone model. When the actual terrain is complex, the cone model can be used to correct the terrain in eight directions. When r=0, it is the original cone model. The laser instrument is used to measure the distance r between the observation point and the slope and the inclination angle α of the slope.

[0059] The cone model calculation formula is as follows:

[0060] (2)

[0061] in, is the gravitational constant 6.67×10-11 m3 / (s2· kg), ρ is the terrain-corrected density 2.67g / cm3, Indicates the distance from the observation point to the cone surface. When n=8, it is an octagonal cone model. is the terrain slope angle.

[0062] On the basis of the original step model, considering that the range of near-area terrain correction is limited, and the formula of the original step model is an infinitely extended step, the present invention derives a step model calculation formula within a circular domain according to the actual situation of near-area terrain correction, and adds a completeness coefficient. The improved near-area terrain correction model is more in line with the actual terrain, and thus can improve the accuracy of near-area terrain correction.

[0063] like Figure 4 As shown in Figure 2, this model is an improved step model. The laser instrument is used to measure the distance r from the gravity measurement point to the step and the height h of the step. At the same time, the integrity coefficient is determined according to the integrity of the step. .

[0064] The calculation formula of the step model is as follows:

[0065] (3)

[0066] in, is the gravitational constant 6.67×10-11 m3 / (s2· kg), ρ is the terrain-corrected density 2.67g / cm3, is the integrity coefficient, with a value range of [-1,1], r1 represents the maximum radius of near-area terrain correction, r is the vertical distance from the gravity measuring point to the step, and H is the relative height difference of the step.

[0067] In the present invention, at the gravity measuring point, a field distance measuring instrument such as a laser rangefinder or a three-dimensional laser scanner is used to obtain the terrain undulation within the near-area terrain correction range (0-20m) of the measuring point, select a suitable near-area terrain correction model or model combination, and record the parameters of the near-area terrain correction model. The near-area terrain correction model includes an improved single slope model, a cone model and a step model.

[0068] The near-zone terrain correction model for each measuring point may be a single near-zone terrain correction model, such as an improved single slope, cone model or step model, or any combination of these three models, or multiple identical near-zone terrain correction models.

[0069] Specifically, the improved single slope model parameters include the inclination angle of the single slope and the distance from the measuring point to the edge of the single slope; the improved cone model parameters include the inclination angle of the cone model and the distance from the measuring point to the edge of the cone model; the improved step model parameters include the height of the step, the distance from the measuring point to the edge of the step and the step integrity coefficient.

[0070] S3: Read the near-area terrain correction entry table and the point number of the gravity measuring point according to the near-area terrain correction model parameters, and calculate the near-area terrain correction value of each gravity measuring point;

[0071] Specifically, step S3 combines the point number of the gravity measuring point, corresponds the near-area terrain correction model parameters at each gravity measuring point one by one and enters them into an electronic form, uses the near-area terrain correction calculation module to read the near-area terrain correction model parameter entry table, and calculates the near-area terrain correction value of each gravity measuring point.

[0072] S4: Obtain DEM data of the exploration area and its vicinity through public channels, and convert them into normal elevation DEM data with an elevation datum of quasi-geoid using the global high-order earth gravity field model;

[0073] As an embodiment, step S4 is specifically as follows:

[0074] S41: Download the DEM data collected by the ALOS (Advanced Land Observing Satellite, launched in 2006) satellite phased array L-band synthetic aperture radar (PALSAR) from NASA's EarthData spatial data network (https: / / search.asf.alaska.edu / ) according to the exploration area and neighborhood range. The effective resolution of the ALOS terrain dataset is about 12.5m. The data has high accuracy and can meet the needs of far-field terrain correction in large-scale gravity surveys. The exploration area and neighborhood range need to be determined based on the radius of far-field terrain correction. The radius of far-field terrain correction for large-scale gravity surveys is 20km.

[0075] S42: The coordinate system of the DEM data downloaded above is WGS84, the projection method is Universal Mercal Projection (UTM), and the data format is GeoTIFF format. Generate a .xyz data format of the WGS84 longitude and latitude coordinate system through splicing, coordinate conversion and file format conversion. Among them, the WGS84 coordinate system (World Geodetic System-1984 CoordinateSystem) is an internationally adopted geocentric coordinate system; the GeoTIFF format is a geographic information image format based on TIFF (Tagged Image File Format); the .xyz data format is a text file that saves three columns of data, of which the first column is the horizontal coordinate, the second column is the vertical coordinate, and the third column is the elevation.

[0076] S43: The global high-order Earth Gravitational Model 2008 (EGM2008) can be used to calculate the height deviation between the mean sea level (MSL) and the surface of the WGS84 coordinate system ellipsoid anywhere on the earth. Therefore, the EGM2008 model is used to convert the downloaded DEM data of the earth's elevation into normal height. The spherical harmonic coefficient order of the EGM2008 model is as high as 2159, which is equivalent to a spatial resolution of about 5′ (about 9km). The mean square error of the converted normal height is less than 1.0 m in the underdeveloped vegetation areas north of the Qinling Mountains in my country, and less than 5.0 m in the developed vegetation areas in southern my country.

[0077] S44: The DEM data after elevation conversion is transformed through plane coordinate projection and projected to plane coordinates consistent with the measured coordinates of the gravity measurement points, so as to prepare for the terrain correction calculation in the mid- and far-field areas.

[0078] S5: Calculate the mid- and far-area terrain correction value of the gravity measuring point based on the converted DEM data and the three-dimensional coordinates of the gravity measuring point;

[0079] It should be noted that in step S5, according to different scale settings, the preset circular area of ​​the central area land correction range is divided into multiple circles with the horizontal coordinates of each gravity measurement point three-dimensional coordinates as the center, the inner ring is divided into 8 directions, the outer ring is divided into 16 directions, and the circle fan column land correction model is used to perform the central area terrain correction calculation;

[0080] In the process of mid- and long-range terrain correction, when the DEM data is divided into azimuths in the circle layer, the azimuth is rotated by a certain angle to obtain the mid- and long-range terrain correction values ​​divided into different rotation angles at a single gravity measurement point, and the average value is taken as the final mid- and long-range terrain correction value of the point.

[0081] As an example, different settings of the scale are as follows:

[0082] It should be noted that when the scale of gravity exploration is greater than 1:50,000, the radius of the circular area of ​​the central area will be 20m to 500m. When the scale of gravity exploration is equal to 1:50,000, the radius of the circular area of ​​the central area will be 20m to 2km. Figure 5 As shown in (a), according to the three-dimensional coordinates of each gravity measuring point, the circular area of ​​the central area is divided into multiple circles (generally 7 circles) with the horizontal coordinates of the measuring point as the center. The inner 4 circles are divided into 8 directions, and the outer 3 circles are divided into 16 directions. The circle fan column model is used to calculate the terrain correction of the central area. The height of each fan column is the difference between the average normal height of all DEM data points within the fan column and the normal height of the gravity measuring point. The height difference is not positive or negative, and the final central area terrain correction is positive.

[0083] It should be noted that when the scale of gravity exploration is greater than 1:50,000, the radius of the circular area of ​​the far-field area is changed from 500m to 20km. When the scale of gravity exploration is equal to 1:50,000, the radius of the circular area of ​​the far-field area is changed from 2km to 20km. Figure 5 As shown in (b) in the figure, according to the three-dimensional coordinates of each gravity measuring point, the circular area of ​​the far-field ground modification range is divided into multiple circles (usually 6 circles) with the horizontal coordinates of the measuring point as the center. The inner three circles are divided into 16 directions, and the outer three circles are divided into 32 directions. Figure 5 In (c), the height of each fan column is the difference between the average normal height of all DEM data points within the fan column and the normal height of the gravity measurement point. The height difference is not positive or negative, and the final far-field ground correction is positive.

[0084] It should be noted that if Figure 6 In the process of mid- and far-field terrain correction, when the DEM data is divided into azimuths in the circle layer, the azimuth is rotated by a certain angle (between 0 and 20°) to obtain the mid- and far-field terrain correction values ​​divided into different rotation angles at a single gravity measurement point, and the average value is taken as the final mid- and far-field terrain correction value of the point.

[0085] S6: Evaluate the accuracy of terrain correction values ​​in near, middle and far areas of large-scale gravity exploration.

[0086] It should be noted that step S6 includes:

[0087] S61: The mean square error of the near-area terrain correction is calculated by quality check points to evaluate the accuracy of the near-area terrain correction. During the large-scale gravity exploration data collection process, the gravity exploration specification requires that the quality check of the measuring points account for no less than 3% of the total number of measuring points. The quality check includes near-area terrain correction, gravity measurement and the three-dimensional coordinates of the measuring points;

[0088] S62: For mid- and far-field terrain correction, when the DEM data is divided into layers in azimuth, the azimuth is rotated by a certain angle (between 0 and 20°) to obtain the mid- and far-field terrain correction values ​​divided into different rotation angles at a single gravity measuring point. The mean square error of the difference between these terrain correction values ​​and the average value is calculated, and the maximum value is taken as the evaluation of the accuracy of the corresponding mid- and far-field terrain correction.

[0089] It should be noted that if Figure 7 The embodiment of the present application conducted error statistics between the measured normal height and the normal height of the collected DEM data in the Yuechengling area of ​​Shaoyang City, Hunan Province. The error statistical results conform to the normal distribution characteristics. The errors are mainly concentrated between -4.0m and 3.0m, and the mean square error is 5.04m.

[0090] It should be noted that if Figure 8 In (a), the embodiment of the present application has carried out statistics on the error between the measured normal height in Dashetai Town, Urad Front Banner, Bayannur City, Inner Mongolia Autonomous Region and the normal height converted from DEM data proposed in the present application. The error is mainly concentrated between -0.75m and -0.50m, and the mean square error is 0.07m. It can be seen that in flat areas with underdeveloped vegetation, the accuracy of normal height converted from DEM data is very high, which can meet the requirements of terrain correction in medium and far areas of large-scale gravity exploration.

[0091] It should be noted that if Figure 8 In (b), the embodiment of the present application carried out statistical analysis of the error between the measured normal height of Lutang Town, Chenzhou City, Hunan Province and the normal height converted from the DEM data proposed in this application. The error statistical results were consistent with the normal distribution characteristics, and the errors were mainly concentrated between -6.0m and 3.0m, with a mean square error of 3.4m.

[0092] According to the comparative analysis of the embodiments, it can be seen that the accuracy of converting DEM elevation through the global high-order earth gravity field model can meet the needs of large-scale gravity exploration in the middle and far areas. On the basis of meeting the accuracy requirements, the technical solution for large-scale gravity exploration in the middle and far areas proposed in this application solves the problems of difficulty in obtaining effective elevation data due to large workload, long time consumption, high labor cost, and vegetation interference factors.

[0093] The present invention also provides a large-scale gravity exploration near, middle and far area terrain correction system, the system comprises: a data entry storage module, a DEM data elevation conversion module, a terrain correction calculation module, and a terrain correction value accuracy statistical analysis module;

[0094] The data entry storage module is connected to the DEM data elevation conversion module; the DEM data elevation conversion module is connected to the terrain correction calculation module; the terrain correction calculation module is connected to the terrain correction value accuracy statistical analysis module;

[0095] The data entry and storage module is used to store the three-dimensional coordinates of each gravity measurement point, near-area terrain correction model parameters and DEM data in real time;

[0096] The DEM data elevation conversion module converts the downloaded earth-height DEM data into normal-height DEM data through a global high-order earth gravity field model;

[0097] The terrain correction calculation module is used to calculate the terrain correction values ​​of the near, middle and far areas at each gravity measuring point according to the near, middle and far area terrain correction model and segmentation scheme determined by each gravity measuring point;

[0098] The terrain correction value accuracy statistical analysis module is used to calculate the mean square error of the difference between the near-area terrain correction values ​​before and after the quality inspection point and the maximum mean square error of the difference between these terrain correction values ​​and the average value calculated by the mid-to-far-area terrain correction subdivision scheme with different azimuth rotation angles, as various accuracy indicators of terrain correction.

[0099] The beneficial effects of the present invention are:

[0100] 1. For the near-area terrain correction of large-scale gravity exploration, the problems existing in the existing methods of near-area terrain correction are fully considered. On the basis of the near-area terrain correction model in the gravity exploration specifications or textbooks, a new near-area terrain correction model that is more suitable for mountainous areas and terraced areas is obtained through theoretical formula derivation. The existence of the new near-area terrain correction model makes the selection of gravity measurement points more practical and principled, which can moderately reduce the impact of near-area terrain correction and improve the accuracy of near-area terrain correction. It greatly reduces the workload and improves work efficiency.

[0101] 2. For the mid-to-far-region terrain correction of large-scale gravity exploration, it is proposed to download the non-dense DEM data of the exploration area and its neighborhood from the official website open to the public, and convert it into normal elevation DEM data through the global high-order earth gravity field model EGM2008 for large-scale mid-to-far-region terrain correction calculation. This not only ensures the accuracy of mid-to-far-region terrain correction, but also solves the problems of difficulty in collecting DEM data, data splicing, and gaps in certain waters.

[0102] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for correcting terrain in near, medium and far areas of large-scale gravity exploration, characterized by: The method comprises the following steps: S1: Obtain the total accuracy requirements of the target Bouguer gravity anomaly and the three-dimensional coordinates of the gravity measurement points in the exploration area; S2: Determine the most suitable near-area terrain correction model or model combination on-site at each gravity measurement point, and obtain the near-area terrain correction model parameters; S3: Read the near-area terrain correction entry table and the point number of the gravity measuring point according to the near-area terrain correction model parameters, and calculate the near-area terrain correction value of each gravity measuring point; S4: Obtain DEM data of the exploration area and its vicinity through public channels, and convert them into normal elevation DEM data with an elevation datum of quasi-geoid using the global high-order earth gravity field model; S5: Calculate the mid- and far-area terrain correction value of the gravity measuring point based on the converted DEM data and the three-dimensional coordinates of the gravity measuring point; S6: Evaluate the accuracy of terrain correction values ​​in near, middle and far areas of large-scale gravity exploration.

2. A method for correcting terrain in near, medium and far areas of large-scale gravity exploration as claimed in claim 1, characterized in that: In step S1, a real-time differential RTK measuring instrument is used to obtain the three-dimensional coordinates of the gravity measuring point.

3. A method for correcting terrain in near, medium and far areas of large-scale gravity exploration as claimed in claim 2, characterized in that: Before step S2, according to the Bouguer total accuracy and the near-area ground modification model parameter table, the near area within the preset area is regarded as flat land and no change is made.

4. A method for correcting terrain in near, medium and far areas of large-scale gravity exploration as claimed in claim 1, characterized in that: The near-area terrain correction model in step S2 includes: an improved single slope model, an improved cone model and an improved step model.

5. A method for correcting terrain in near, medium and far areas of large-scale gravity exploration as claimed in claim 4, characterized in that: The improved single slope model and improved cone model have increased distance parameters, and the improved step model has increased integrity coefficients.

6. A method for correcting terrain in near, medium and far areas of large-scale gravity exploration as claimed in claim 5, characterized in that: In step S5, according to different scale settings, the preset circular area of ​​the central area terrain correction range is divided into multiple circles with the horizontal coordinates of each gravity measurement point three-dimensional coordinate as the center, the inner circle is divided into 8 directions, the outer circle is divided into 16 directions, and the circle fan column terrain correction model is used to perform the central area terrain correction calculation; In the process of mid- and long-range terrain correction, when the DEM data is divided into azimuths in the circle layer, the azimuth is rotated by a certain angle to obtain the mid- and long-range terrain correction values ​​divided into different rotation angles at a single gravity measurement point, and the average value is taken as the final mid- and long-range terrain correction value of the point.

7. A method for correcting terrain in near, medium and far areas of large-scale gravity exploration as claimed in claim 4, characterized in that: Step S6 is specifically as follows: S61: The mean square error of the near-area terrain correction is calculated by means of quality check points to evaluate the accuracy of the near-area terrain correction; S62: For mid- and far-field terrain correction, when the DEM data is divided into layers, the azimuth is rotated by a certain angle to obtain the mid- and far-field terrain correction values ​​divided into different rotation angles at a single gravity measurement point. The mean square error of the difference between these terrain correction values ​​and the average value is calculated, and the maximum value is taken as the evaluation of the accuracy of the corresponding mid- and far-field terrain correction.

8. A large-scale gravity exploration near, medium and far area terrain correction system, characterized by: include: Data entry and storage module, DEM data elevation conversion module, terrain correction calculation module, terrain correction value accuracy statistical analysis module; The data entry storage module is connected to the DEM data elevation conversion module; the DEM data elevation conversion module is connected to the terrain correction calculation module; the terrain correction calculation module is connected to the terrain correction value accuracy statistical analysis module; The data entry and storage module is used to store the three-dimensional coordinates of each gravity measurement point, near-area terrain correction model parameters and DEM data in real time; The DEM data elevation conversion module converts the downloaded earth-height DEM data into normal-height DEM data through a global high-order earth gravity field model; The terrain correction calculation module is used to calculate the terrain correction values ​​of the near, middle and far areas at each gravity measurement point according to the near, middle and far area terrain correction model and segmentation scheme determined by each gravity measurement point; the process of the terrain correction calculation module is as follows: Determine the most suitable near-area terrain correction model or model combination on-site at each gravity measurement point, and obtain the near-area terrain correction model parameters; Read the near-area terrain correction entry table and the gravity point number according to the near-area terrain correction model parameters, and calculate the near-area terrain correction value of each gravity point; According to the converted DEM data and the three-dimensional coordinates of the gravity measuring point, the mid- and far-area terrain correction value of the gravity measuring point is calculated; The terrain correction value accuracy statistical analysis module is used to calculate the mean square error of the difference between the near-area terrain correction values ​​before and after the quality inspection point and the maximum mean square error of the difference between these terrain correction values ​​and the average value calculated by the mid-to-far-area terrain correction subdivision scheme with different azimuth rotation angles, as various accuracy indicators of terrain correction.

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